Tungsten heavy metal alloy powders and methods of forming them

ABSTRACT

In various embodiments, metallic alloy powders are formed at least in part by spray drying to form agglomerate particles and/or plasma densification to form composite particles.

RELATED APPLICATION

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 62/432,080, filed Dec. 9, 2016, the entiredisclosure of which is hereby incorporated herein by reference.

TECHNICAL FIELD

In various embodiments, the present invention relates to metal powdersand the formation thereof, and in particular to metal powders includingat least one refractory metal.

BACKGROUND

Additive manufacturing, or three-dimensional (3D) printing, is a widelyutilized technique for rapid manufacturing and rapid prototyping. Ingeneral, additive manufacturing involves the layer-by-layer depositionof material by computer control to form a three-dimensional object. Mostadditive manufacturing techniques to date have utilized polymeric orplastic materials as raw materials, as such materials are easily handledand melt at low temperatures. While various techniques have beenutilized for the additive manufacturing of metallic parts, metallicprecursor materials present a host of challenges, particularly when thedesired material is an alloy or mixture of different elemental metals.Conventional techniques, when utilized with metallic precursormaterials, may result in parts of inadequate density or that do not meetASTM specifications. Thus, there is a need for techniques for thepreparation of metallic alloy precursors (e.g., in the form of powders)which, when utilized with suitable additive manufacturing techniques,result in the formation of a highly densified metallic part.

SUMMARY

In accordance with various embodiments of the present invention, aprecursor material including, consisting essentially of, or consistingof a metallic alloy is formed as a highly flowable, highly dense powderof composite particles that is suitable for additive manufacturing. Invarious embodiments, an initial powder is prepared by blending powdersof the various elemental metal constituents of the desired metal alloytogether. A slurry is formed by mixing the powder blend with one or moreliquids (e.g., water and/or one or more organic binders), and then theslurry is spray-dried to produce flowable agglomerate particles (i.e.,particles each including, consisting essentially of, or consisting of amixture or alloy of the precursor metals, rather than each particlebeing composed of a single elemental metal as in the initial powderblend). The agglomerate is thermally heated (i.e., sintered) to removeany organic material and to densify the agglomerate. In some embodimentsof the invention, the agglomerate particles may be plasma densified inorder to further increase their density. The resulting densifiedcomposite particles are highly flowable (e.g., as measured with a Hallflowmeter), thereby enabling the powder to be reliably fed through apowder feeder and/or spread uniformly over a powder bed for additivemanufacturing. The composite particles in accordance with embodiments ofthe invention also have a high density, which minimizes shrinkage (i.e.,volume reduction) during subsequent melting and/or sintering processes.High-density powder utilized in a powder bed may also improve thermalconductivity of the powder bed. In some embodiments, the powder ofcomposite particles has a density of approximately 35%-approximately 65%of the theoretical density of the target metallic alloy. The compositeparticles also have low (e.g., non-zero) concentrations of, or aresubstantially free of, interstitial and surface contaminants, thepresence of which may compromise the mechanical properties of the final3D-printed part (e.g., lead to increased porosity).

In accordance with embodiments of the invention, the alloy compositeparticles are utilized to form a 3D part by additive manufacturing. Inan exemplary embodiment, a printing head is utilized to disperse aliquid binder or adhesive (typically a polymer material) into a powderbed of the composite particles layer-by-layer in approximately thedesired shape and size for the part. After each layer, the binder may becured by, e.g., application of heat or light. After the printing iscomplete, the shaped, 3D part is made of the composite particles heldtogether by the binder material. The shaped part may then be sintered tofuse the particles together and decompose (i.e., burn off) some or allof the binder material and possibly leave empty pores (if desired; suchpores may subsequently be infiltrated with another material by placingthe shaped part in contact with the material and heat treating thearticle such that the material infiltrates into the pores of the shapedpart).

As utilized herein, the term “substantially spherical” means sphericalto within ±10%, and in some embodiments, ±5% in any direction—i.e., theeccentricity in any direction does not exceed 5% or 10%. As utilizedherein, “non-spherical” means elongated with an aspect ratio of at least2:1, acicular, having at least one flat surface (e.g., a flake with twoopposed flat surfaces), having at least one corner or vertex, orpolyhedral.

In an aspect, embodiments of the invention feature a powder for additivemanufacturing of a part that includes, consists essentially of, orconsists of a tungsten heavy alloy. The tungsten heavy alloy includes,consists essentially of, or consists of approximately 90% or moretungsten and 10% or less (or between approximately 1% and approximately10%, or between approximately 0.5% and approximately 10%) of one or moreadditional elements selected from the group consisting of nickel, iron,copper, cobalt, and manganese, and may also include trace amounts ofimpurities. For example, the concentrations of elements such as oxygen(O), sodium (Na), magnesium (Mg), phosphorus (P), sulfur (S), potassium(K), calcium (Ca), and/or antimony (Sb) may be present at a level belowa concentration of 20 ppm, below a concentration of 10 ppm, below aconcentration of 5 ppm, below a concentration of 3 ppm, below aconcentration of 2 ppm, or even below a concentration of 1 ppm, and maybe present at a concentration of at least 0.01 ppm, at least 0.05 ppm,at least 0.1 ppm, or even at least 0.2 ppm (all concentrations hereinare by weight unless otherwise indicated). The tungsten heavy alloy hasa theoretical density corresponding to a weighted average of thedensities of tungsten and the one or more additional elements. Thepowder includes, consists essentially of, or consists of a plurality ofsubstantially spherical composite particles. Each composite particleincludes, consists essentially of, or consists of a plurality oftungsten grains surrounded by a matrix including, consisting essentiallyof, or consisting of at least one of the one or more additionalelements. The bulk density of the powder may be approximately 35% ormore of the theoretical density. The bulk density of the powder may beapproximately 80% or less of the theoretical density. The tap density ofthe powder may range from approximately 40% to approximately 75% of thetheoretical density.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The Hall flow rate of the powder mayrange from approximately 1 s/50 g to approximately 15 s/50 g. The powdermay have a particle-size distribution d10 between 2 microns and 8microns, d50 between 15 microns and 25 microns, and d90 between 50microns and 70 microns, where a particle-size distribution dX of Ydenotes that X % of particles have a size less than Y. The bulk densityof the powder may be approximately 45% or more of the theoreticaldensity. The bulk density of the powder may be approximately 50% or moreof the theoretical density. The bulk density of the powder may beapproximately 65% or less of the theoretical density.

In another aspect, embodiments of the invention feature a method offorming a powder that includes, consists essentially of, or consists ofa tungsten heavy alloy. The tungsten heavy alloy (i) includes, consistsessentially of, or consists of 90% or more tungsten and 10% or less ofone or more additional elements selected from the group consisting ofnickel, iron, copper, cobalt, and manganese, and (ii) has a theoreticaldensity corresponding to a weighted average of the densities of tungstenand the one or more additional elements. A powder blend is formed byblending together powders of tungsten and the one or more additionalelements. Each powder may be an elemental metal powder consistingessentially of or consisting of tungsten or one of the other elements. Aslurry is formed by mixing at least a portion of the powder blend with aliquid. The liquid includes, consists essentially of, or consists ofwater and/or one or more organic binders. At least a portion of theslurry and a heated gas are sprayed into a drying chamber to form aplurality of agglomerate particles each including, consistingessentially of, or consisting of a mixture of tungsten and at least oneof the one or more additional elements. At least some of the agglomerateparticles are densified to form the powder. The densification includes,consists essentially of, or consists of heating to a temperature greaterthan a melting point of at least one of the additional elements and lessthan a melting point of tungsten. The powder includes, consistsessentially of, or consists of a plurality of substantially sphericalcomposite particles, each composite particle including, consistingessentially of, or consisting of a plurality of tungsten grainssurrounded by a matrix including, consisting essentially of, orconsisting of at least one of the one or more additional elements.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The densification may include, consistessentially of, or consist of feeding the at least a portion of theplurality of agglomerate particles through a plasma. The Hall flow rateof the powder may range from approximately 1 s/50 g to approximately 15s/50 g. The powder may have a particle-size distribution d10 between 2microns and 8 microns, d50 between 15 microns and 25 microns, and d90between 50 microns and 70 microns, where a particle-size distribution dXof Y denotes that X % of particles have a size less than Y. The bulkdensity of the powder may be approximately 45% or more of thetheoretical density. The bulk density of the powder may be approximately50% or more of the theoretical density. The bulk density of the powdermay be approximately 65% or less of the theoretical density.

In yet another aspect, embodiments of the invention feature a powder foradditive manufacturing of a part that includes, consists essentially of,or consists of a metallic alloy containing one or more refractory metalsand one or more transition metals having a melting point less than thatof the one or more refractory metals. The alloy may include, consistessentially of, or consist of 60% or more, 70% or more, 80% or more, or90% or more of the one or more refractory metals and 40% or less, 30% orless, 20% or less, or 10% or less of the one or more transition metals,and may also include trace amounts of impurities. For example, theconcentrations of elements such as O, Na, Mg, P, S, K, Ca, and/or Sb maybe present at a level below a concentration of 20 ppm, below aconcentration of 10 ppm, below a concentration of 5 ppm, below aconcentration of 3 ppm, below a concentration of 2 ppm, or even below aconcentration of 1 ppm, and may be present at a concentration of atleast 0.01 ppm, at least 0.05 ppm, at least 0.1 ppm, or even at least0.2 ppm (all concentrations herein are by weight unless otherwiseindicated). The metal alloy has a theoretical density corresponding to aweighted average of the densities of the one or more refractory metalsand the one or more transition metals. The powder includes, consistsessentially of, or consists of a plurality of substantially sphericalcomposite particles. Each composite particle includes, consistsessentially of, or consists of a plurality of grains surrounded by amatrix including, consisting essentially of, or consisting of at leastone of the one or more transition metals. The grains include, consistessentially of, or consist of at least one of the one or more refractorymetals. The bulk density of the powder may be approximately 35% or moreof the theoretical density. The bulk density of the powder may beapproximately 80% or less of the theoretical density. The tap density ofthe powder may range from approximately 40% to approximately 75% of thetheoretical density.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The Hall flow rate of the powder mayrange from approximately 1 s/50 g to approximately 15 s/50 g. The powdermay have a particle-size distribution d10 between 2 microns and 8microns, d50 between 15 microns and 25 microns, and d90 between 50microns and 70 microns, where a particle-size distribution dX of Ydenotes that X % of particles have a size less than Y. The bulk densityof the powder may be approximately 45% or more of the theoreticaldensity. The bulk density of the powder may be approximately 50% or moreof the theoretical density. The bulk density of the powder may beapproximately 65% or less of the theoretical density.

In another aspect, embodiments of the invention feature a method offorming a powder that includes, consists essentially of, or consists ofa metallic alloy containing one or more refractory metals and one ormore transition metals having a melting point less than that of the oneor more refractory metals. The alloy may include, consist essentiallyof, or consist of 60% or more, 70% or more, 80% or more, or 90% or moreof the one or more refractory metals and 40% or less, 30% or less, 20%or less, or 10% or less of the one or more transition metals, and mayalso include trace amounts of impurities. For example, theconcentrations of elements such as O, Na, Mg, P, S, K, Ca, and/or Sb maybe present at a level below a concentration of 20 ppm, below aconcentration of 10 ppm, below a concentration of 5 ppm, below aconcentration of 3 ppm, below a concentration of 2 ppm, or even below aconcentration of 1 ppm, and may be present at a concentration of atleast 0.01 ppm, at least 0.05 ppm, at least 0.1 ppm, or even at least0.2 ppm (all concentrations herein are by weight unless otherwiseindicated). The metal alloy has a theoretical density corresponding to aweighted average of the densities of the one or more refractory metalsand the one or more transition metals. A powder blend is formed byblending together powders of the one or more refractory metals and theone or more transition metals. Each powder may be an elemental metalpowder consisting essentially of or consisting of one of the refractorymetals or one of the transition metals. A slurry is formed by mixing atleast a portion of the powder blend with a liquid. The liquid includes,consists essentially of, or consists of water and/or one or more organicbinders. At least a portion of the slurry and a heated gas are sprayedinto a drying chamber to form a plurality of agglomerate particles eachincluding, consisting essentially of, or consisting of a mixture of theone or more refractory metals and the one or more transition metals. Atleast some of the agglomerate particles are densified to form thepowder. The densification includes, consists essentially of, or consistsof heating to a temperature greater than a melting point of at least one(or even all) of the transition metals and less than a melting point ofat least one (or even all) of the refractory metals. The powderincludes, consists essentially of, or consists of a plurality ofsubstantially spherical composite particles, each composite particleincluding, consisting essentially of, or consisting of a plurality ofgrains surrounded by a matrix including, consisting essentially of, orconsisting of at least one of the one or more transition metals. Thegrains include, consist essentially of, or consist of at least one ofthe one or more refractory metals.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The densification may include, consistessentially of, or consist of feeding the at least a portion of theplurality of agglomerate particles through a plasma. The Hall flow rateof the powder may range from approximately 1 s/50 g to approximately 15s/50 g. The powder may have a particle-size distribution d10 between 2microns and 8 microns, d50 between 15 microns and 25 microns, and d90between 50 microns and 70 microns, where a particle-size distribution dXof Y denotes that X % of particles have a size less than Y. The bulkdensity of the powder may be approximately 45% or more of thetheoretical density. The bulk density of the powder may be approximately50% or more of the theoretical density. The bulk density of the powdermay be approximately 65% or less of the theoretical density.

In another aspect, embodiments of the invention feature a method offabricating a three-dimensional object that includes, consistsessentially of, or consists of a metallic alloy including, consistingessentially of, or consisting of a plurality of constituent metals(e.g., elemental metals). A powder blend is formed by blending togetherpowders of each of the constituent metals. A slurry is formed by mixingthe powder blend with a liquid, the liquid comprising water and/or oneor more organic binders. The slurry and one or more heated gases aresprayed into a drying chamber to form a plurality of agglomerateparticles each including, consisting essentially of, or consisting of amixture of the plurality of constituent metals. A powder bed containingthe particles is provided. A first layer of a shaped part is formed by(i) dispersing a binder into the powder bed, and (ii) curing the binder.The first layer of the shaped part includes, consists essentially of, orconsists of particles bound together by cured binder. A layer of theparticles is disposed or dispersed over the first layer of the shapedpart. Subsequent layers of the shaped part are formed by (i) dispersingbinder over the particles, and (ii) curing the binder, additionalparticles being disposed over the shaped part between layers. The shapedpart is sintered to burn away at least a portion (or even substantiallyall) of the cured binder to form the three-dimensional object.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. At least some of the plurality ofagglomerate particles may be densified (e.g., by plasma densification)prior to forming the shaped part to form composite particles. Some orall of the particles provided in the powder bed may be compositeparticles. Each of the composite particles may include, consistessentially of, or consist of a plurality of grains surrounded by amatrix. The grains may include, consist essentially of, or consist ofone or more of the metals (e.g., the one or more metals having thehighest melting points), and the matrix may include, consist essentiallyof, or consist of one or more of the others of the metals (e.g., the oneor more metals having the lowest melting points). The grains mayinclude, consist essentially of, or consist of one or more refractorymetals. The matrix may include, consist essentially of, or consist ofone or more transition metals. The grains may include, consistessentially of, or consist of tungsten. The matrix may include, consistessentially of, or consist of one or more of nickel, iron, copper,cobalt, or manganese. The alloy may include, consist essentially of, orconsist of 60% or more, 70% or more, 80% or more, or 90% or more of theone or more refractory metals and 40% or less, 30% or less, 20% or less,or 10% or less of the one or more transition metals, and may alsoinclude trace amounts of impurities. For example, the concentrations ofelements such as O, Na, Mg, P, S, K, Ca, and/or Sb may be present at alevel below a concentration of 20 ppm, below a concentration of 10 ppm,below a concentration of 5 ppm, below a concentration of 3 ppm, below aconcentration of 2 ppm, or even below a concentration of 1 ppm, and maybe present at a concentration of at least 0.01 ppm, at least 0.05 ppm,at least 0.1 ppm, or even at least 0.2 ppm (all concentrations hereinare by weight unless otherwise indicated). The alloy may include,consist essentially of, or consist of 60% or more, 70% or more, 80% ormore, or 90% or more of tungsten and 40% or less, 30% or less, 20% orless, or 10% or less of one or more of nickel, iron, copper, cobalt, ormanganese, and may also include trace amounts of impurities. The shapedpart may be sintered in an atmosphere or ambient that includes, consistsessentially of, or consists of hydrogen (e.g., a mixture of hydrogen andnitrogen). The shaped part may be sintered at a temperature rangingbetween approximately 1400° C. and approximately 1500° C. Duringformation of each layer of the shaped part, the binder may be cured viaapplication of light and/or heat.

In yet another aspect, embodiments of the invention feature a method offabricating a three-dimensional object that includes, consistsessentially of, or consists of a metallic alloy including, consistingessentially of, or consisting of a plurality of constituent metals(e.g., elemental metals). A powder bed containing particles is provided.The particles each include, consist essentially of, or consist of amixture or alloy of two or more (or even all) of the constituent metals.The particles are formed at least in part by spray drying of a slurrycontaining a blend of powders of each of the constituent metals. A firstlayer of a shaped part is formed by (i) dispersing a binder into thepowder bed, and (ii) curing the binder. The first layer of the shapedpart includes, consists essentially of, or consists of particles boundtogether by cured binder. A layer of the particles is disposed ordispersed over the first layer of the shaped part. Subsequent layers ofthe shaped part are formed by (i) dispersing binder over the particles,and (ii) curing the binder, additional particles being disposed over theshaped part between layers. The shaped part is sintered to burn away atleast a portion (or even substantially all) of the cured binder to formthe three-dimensional object.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The particles may be formed in part bydensification (e.g., plasma densification) after spray drying. Each ofthe particles may include, consist essentially of, or consist of aplurality of grains surrounded by a matrix. The grains may include,consist essentially of, or consist of one or more of the metals (e.g.,the one or more metals having the highest melting points), and thematrix may include, consist essentially of, or consist of one or more ofthe others of the metals (e.g., the one or more metals having the lowestmelting points). The grains may include, consist essentially of, orconsist of one or more refractory metals. The matrix may include,consist essentially of, or consist of one or more transition metals. Thegrains may include, consist essentially of, or consist of tungsten. Thematrix may include, consist essentially of, or consist of one or more ofnickel, iron, copper, cobalt, or manganese. The alloy may include,consist essentially of, or consist of 60% or more, 70% or more, 80% ormore, or 90% or more of the one or more refractory metals and 40% orless, 30% or less, 20% or less, or 10% or less of the one or moretransition metals, and may also include trace amounts of impurities. Forexample, the concentrations of elements such as O, Na, Mg, P, S, K, Ca,and/or Sb may be present at a level below a concentration of 20 ppm,below a concentration of 10 ppm, below a concentration of 5 ppm, below aconcentration of 3 ppm, below a concentration of 2 ppm, or even below aconcentration of 1 ppm, and may be present at a concentration of atleast 0.01 ppm, at least 0.05 ppm, at least 0.1 ppm, or even at least0.2 ppm (all concentrations herein are by weight unless otherwiseindicated). The alloy may include, consist essentially of, or consist of60% or more, 70% or more, 80% or more, or 90% or more of tungsten and40% or less, 30% or less, 20% or less, or 10% or less of one or more ofnickel, iron, copper, cobalt, or manganese, and may also include traceamounts of impurities. The shaped part may be sintered in an atmosphereor ambient that includes, consists essentially of, or consists ofhydrogen (e.g., a mixture of hydrogen and nitrogen). The shaped part maybe sintered at a temperature ranging between approximately 1400° C. andapproximately 1500° C. During formation of each layer of the shapedpart, the binder may be cured via application of light and/or heat.

In another aspect, embodiments of the invention feature a method offabricating a three-dimensional object that includes, consistsessentially of, or consists of a metallic alloy including, consistingessentially of, or consisting of a plurality of constituent metals(e.g., elemental metals). A powder bed containing particles is provided.The particles each include, consist essentially of, or consist of amixture and/or alloy of two or more (or even all) of the constituentmetals. A first layer of a shaped part is formed by (i) dispersing abinder into the powder bed, and (ii) curing the binder. The first layerof the shaped part includes, consists essentially of, or consists ofparticles bound together by cured binder. A layer of the particles isdisposed or dispersed over the first layer of the shaped part.Subsequent layers of the shaped part are formed by (i) dispersing binderover the particles, and (ii) curing the binder, additional particlesbeing disposed over the shaped part between layers. The shaped part issintered to burn away at least a portion (or even substantially all) ofthe cured binder to form the three-dimensional object.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. At least some of the particles may bespray-dried agglomerate particles. At least some of the particles may bedensified, substantially spherical composite particles. Each of theparticles may include, consist essentially of, or consist of a pluralityof grains surrounded by a matrix. The grains may include, consistessentially of, or consist of one or more of the metals (e.g., the oneor more metals having the highest melting points), and the matrix mayinclude, consist essentially of, or consist of one or more of the othersof the metals (e.g., the one or more metals having the lowest meltingpoints). The grains may include, consist essentially of, or consist ofone or more refractory metals. The matrix may include, consistessentially of, or consist of one or more transition metals. The grainsmay include, consist essentially of, or consist of tungsten. The matrixmay include, consist essentially of, or consist of one or more ofnickel, iron, copper, cobalt, or manganese. The alloy may include,consist essentially of, or consist of 60% or more, 70% or more, 80% ormore, or 90% or more of the one or more refractory metals and 40% orless, 30% or less, 20% or less, or 10% or less of the one or moretransition metals, and may also include trace amounts of impurities. Forexample, the concentrations of elements such as O, Na, Mg, P, S, K, Ca,and/or Sb may be present at a level below a concentration of 20 ppm,below a concentration of 10 ppm, below a concentration of 5 ppm, below aconcentration of 3 ppm, below a concentration of 2 ppm, or even below aconcentration of 1 ppm, and may be present at a concentration of atleast 0.01 ppm, at least 0.05 ppm, at least 0.1 ppm, or even at least0.2 ppm (all concentrations herein are by weight unless otherwiseindicated). The alloy may include, consist essentially of, or consist of60% or more, 70% or more, 80% or more, or 90% or more of tungsten and40% or less, 30% or less, 20% or less, or 10% or less of one or more ofnickel, iron, copper, cobalt, or manganese, and may also include traceamounts of impurities. The shaped part may be sintered in an atmosphereor ambient that includes, consists essentially of, or consists ofhydrogen (e.g., a mixture of hydrogen and nitrogen). The shaped part maybe sintered at a temperature ranging between approximately 1400° C. andapproximately 1500° C. During formation of each layer of the shapedpart, the binder may be cured via application of light and/or heat.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “approximately” and “substantially” mean±10%, and in someembodiments, ±5%. The term “consists essentially of” means excludingother materials that contribute to function, unless otherwise definedherein. Nonetheless, such other materials may be present, collectivelyor individually, in trace amounts. For example, a structure consistingessentially of multiple metals will generally include only those metalsand only unintentional impurities (which may be metallic ornon-metallic) that may be present in non-zero concentrations and/ordetectable via chemical analysis but do not contribute to function. Asused herein, “consisting essentially of at least one metal” refers to ametal or a mixture of two or more metals but not compounds between ametal and a non-metallic element or chemical species such as oxygen,silicon, or nitrogen (e.g., metal nitrides, metal silicides, or metaloxides); such non-metallic elements or chemical species may be present,collectively or individually, in trace amounts, e.g., as impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 is a schematic diagram of a spray drying apparatus utilized toform agglomerate particles in accordance with various embodiments of theinvention;

FIG. 2 is a schematic cross-sectional view of a plasma densificationapparatus utilized to form substantially spherical densified powderparticles in accordance with various embodiments of the invention;

FIG. 3A is a micrograph depicting tungsten heavy alloy agglomerateparticles fabricated in accordance with various embodiments of theinvention;

FIG. 3B is a micrograph depicting tungsten heavy alloy densifiedcomposite particles fabricated in accordance with various embodiments ofthe invention;

FIG. 4 is a micrograph depicting grain structure of tungsten heavy alloydensified composite particles fabricated in accordance with variousembodiments of the invention;

FIG. 5 is a schematic of an additive-manufacturing apparatus utilized tofabricate three-dimensional metallic parts in accordance with variousembodiments of the invention;

FIG. 6 is a cross-sectional micrograph of a three-dimensional metallicpart fabricated from spray-dried agglomerate particles in accordancewith various embodiments of the invention;

FIG. 7 is a cross-sectional micrograph of a three-dimensional metallicpart fabricated from plasma-densified composite particles in accordancewith various embodiments of the invention; and

FIG. 8 is a schematic of an additive-manufacturing apparatus utilized tofabricate three-dimensional metallic parts in accordance with variousembodiments of the invention.

DETAILED DESCRIPTION

The ensuing discussion focuses on tungsten heavy alloys as exemplarymaterials, but it should be understood that the invention has broadapplicability to other metallic alloys, particularly those containingone or more refractory metals. For example, metallic alloys inaccordance with embodiments of the invention include one or morerefractory metals such as niobium (Nb), tantalum (Ta), rhenium (Re),tungsten (W), and/or molybdenum (Mo). Such alloys may also include oneor more transition metals (e.g., one or more of copper (Cu), cobalt(Co), nickel (Ni), iron (Fe), manganese (Mn), silver (Ag), gold (Au),cadmium (Cd), vanadium (V), palladium (Pd), zirconium (Zr), or yttrium(Y)) and/or one or more other metals having a melting point lower thanthat of the one or more refractory metals.

In an exemplary embodiment, spray drying is utilized to form acollection (or “powder”) of agglomerate particles that include, consistessentially of, or consist of a mixture of elements constituting atungsten heavy alloy i.e., a mixture or alloy that includes more thanapproximately 90% tungsten (W) and also includes small amounts (e.g.,approximately 1% to approximately 10%) of metal additives, e.g., one ormore transition metals such as nickel (Ni), iron (Fe), copper (Cu),cobalt (Co), and/or manganese (Mn). Specific examples of tungsten heavyalloys include (all percentages are by weight unless otherwiseindicated) (i) 90% W, 6% Ni, 4% Cu, (ii) 90% W, 7% Ni, 3% Fe, (iii)92.5% W, 5.25% Ni, 2.25% Fe, (iv) 95% W, 3.5% Ni, 1.5% Cu, (v) 95% W,3.5% Ni, 1.5% Fe, (vi) 97% W, 2.1% Ni, 0.9% Fe, (vii) 90% W, 5% Ni, 3%Fe, 2% Cu. As detailed herein, the spray-dried agglomerate particles maybe advantageously densified by, e.g., plasma densification, to formcomposite particles that each includes, consists essentially of, orconsists of a mixture and/or alloy of the mixture of elements within theagglomerate particles (and hence, within the initial powder blend).

While the composite particles each typically include, consistessentially of, or consist of a mixture of two or more (or even all) ofthe different elements of the desired alloy, the individual compositeparticles do not necessarily have the same composition. That is, invarious embodiments there is a distribution of the compositions of theindividual composite particles, but when considered as a large group,any compositional differences tend to average out to the desired alloycomposition overall. In various embodiments of the invention, thecomposite particles each include, consist essentially of, or consist ofa mixture of two or more elements, where a primary element is present atapproximately 50% or more (e.g., approximately 70% or more,approximately 80% or more, or even approximately 90% or more) and has amelting point significantly higher than the melting points or one ormore secondary elements, each of which are present at less than 50%(e.g., approximately 40% or less, approximately 30% or less,approximately 20% or less, approximately 10% or less, approximately 5%or less, approximately 2% or less, or even approximately 1% or less).For example, the melting point of the primary element may be at leastapproximately 400° C., at least approximately 700° C., or even at leastapproximately 1000° C. higher than the melting point of one or more of(or even all of) the secondary element(s).

In other embodiments, the composition of each of the composite particlesis substantially equal to the composition of the desired target alloy(i.e., the composition of each composite particle is approximately thesame as the overall composition of the collection of composite particlesconsidered in volume).

In various embodiments of the invention, the powder of compositeparticles is highly flowable, as measured in accordance with ASTMB213-13, Standard Test Methods for Flow Rate of Metal Powders Using theHall Flowmeter Funnel, ASTM International, West Conshohocken, Pa., 2013,the entire disclosure of which is incorporated by reference herein. Inaccordance therewith, an AS-300 Hall Flowmeter Funnel may be utilized todetermine Hall flow rate by measuring the time taken by 50 grams of thepowder to flow through the Hall flowmeter funnel. For example, inaccordance with embodiments of the present invention, the alloy powderparticles may have a Hall flow rate between approximately 5 s/50 g andapproximately 50 s/50 g. In various embodiments, aggregate particlesthat have been spray dried but not densified may have a Hall flow ratebetween approximately 25 s/50 g and approximately 50 s/50 g. In variousembodiments, densified composite particles that have been plasmadensified may have a Hall flow rate between approximately 1 s/50 g andapproximately 25 s/50 g, between approximately 1 s/50 g andapproximately 15 s/50 g, between approximately 1 s/50 g andapproximately 10 s/50 g, between approximately 2 s/50 g andapproximately 10 s/50 g, or between approximately 5 s/50 g andapproximately 10 s/50 g.

In various embodiments of the invention, the tap (or “tapped”) densityof the powder of composite particles is between approximately 25% andapproximately 75% of the theoretical density of the material, asmeasured in accordance with ASTM B527-15, Standard Test Method for TapDensity of Metal Powders and Compounds, ASTM International, WestConshohocken, Pa., 2015, the entire disclosure of which is incorporatedby reference herein. In various embodiments, the tap density of thecomposite particles is between approximately 35% and approximately 75%of the theoretical density of the material, or even betweenapproximately 40% and approximately 70% of the theoretical density ofthe material. In various embodiments, the tap density of the compositeparticles is approximately 50% of the theoretical density of thematerial. In various embodiments, the tap density of the compositeparticles is between approximately 2% and approximately 10% larger thanthe bulk density of the composite particles, or between approximately 3%and approximately 8% larger than the bulk density of the compositeparticles, or between approximately 4% and approximately 7% larger thanthe bulk density of the composite particles, or approximately 5% largerthan the bulk density of the composite particles.

In various embodiments of the invention, the composite particles alsohave low concentrations of interstitial impurities and surfacecontaminants. Such purity typically results in improved mechanicalproperties of the final printed part and reduced porosity from volatilecompounds. For example, the composite particles may have oxygenconcentrations less than approximately 1000 ppm, less than approximately750 ppm, less than approximately 500 ppm, less than approximately 250ppm, less than approximately 100 ppm, or less than approximately 50 ppm.The oxygen concentration may be non-zero, e.g., at least approximately0.5 ppm, at least approximately 1 ppm, at least approximately 2 ppm, atleast approximately 5 ppm, or at least approximately 10 ppm. Thecomposite particles may also have carbon concentrations less thanapproximately 500 ppm, less than approximately 250 ppm, less thanapproximately 100 ppm, or less than approximately 50 ppm. The carbonconcentration may be non-zero, e.g., at least approximately 0.5 ppm, atleast approximately 1 ppm, at least approximately 2 ppm, at leastapproximately 5 ppm, or at least approximately 10 ppm. In variousembodiments, densified composite particles are also substantiallyspherical after plasma densification.

In accordance with embodiments of the invention, a powder blend isinitially formed by combining powders of the individual elemental metalsof the desired alloy. The powder blend is then mixed with water and/orone or more organic binders to form a slurry. A dry powder issubsequently formed from the slurry. For example, the slurry may bespray dried to form agglomerate particles that each include, consistessentially of, or consist of the desired alloy (and/or a mixture of theelements within the alloy). An exemplary spray drying system 100 isshown in FIG. 1. As shown, the slurry is fed from a slurry feeder 105 toan atomizer or spray nozzle 110, which disperses the slurry into adrying chamber 115. A heat source 120 is utilized to heat air (and/orone or more other gases such as nitrogen and/or an inert gas such asargon) that is also introduced into the drying chamber 115 to dry theslurry. The drying gas flows out of the drying chamber 115, e.g., drawnout by suction caused by a pump or fan 125, and any dust is filtered outof the gas and collected at one or more dust collectors 130. The dryagglomerate particles exit the drying chamber 115 via a collection port135 and are collected below the drying chamber 115. The particles maysubsequently be screened and/or air classified to select particles of aspecific size range.

In various embodiments of the invention, the spray-dried agglomerateparticles are advantageously plasma densified for enhanced densificationand/or shaping (i.e., to become substantially spherical). An exemplaryapparatus 200 for plasma densification is shown schematically in FIG. 2.As shown, agglomerate powder particles 210 may be loaded into a powderfeeder 220, which feeds the particles 210 through a plasma jet 230formed by, for example, a time-varying current applied to an inductioncoil 240 sparking a plasma 250 from plasma gas 260 fed into the coil240. The plasma jet 230 at least partially melts the particles 210,which subsequently resolidify into higher-density composite particles270 collected below the plasma 250. The plasma-densified compositeparticles 270 are generally substantially spherical due to theplasma-induced melting and minimization of surface area resulting duringresolidification. The minimization of the surface area of the particlesalso minimizes or substantially reduces the uptake of oxygen or othervolatile species, and the plasma densification process itselfvolatilizes such species as well, thereby reducing the concentration ofsuch contaminants within the powder 270. The plasma-densified compositepowder particles 270 may have an average particle size of, for example,15 μm to 45 μm, or even smaller.

In other embodiments, the composite powder particles may be formed viafeeding the agglomerate powder particles through the plasma jetgenerated by a commercially available plasma torch. In variousembodiments, this may enable lower-temperature formation of thecomposite particles, which may advantageously result in less orsubstantially no loss (i.e., due to vaporization) of lower-melting-pointalloying elements in the powder particles during plasma densification.The operating temperature of the plasma torch may be decreased by, e.g.,increasing the gas flow rate of the gas utilized to form the plasma,decreasing the operating current (or operating power), or use of aplasma-formation gas composed of more complex molecules (for example,use of a more complex gas molecule such as methane may result in lowerplasma temperatures than use of a gas such as argon).

The table below presents various parameters of tungsten heavy metalalloy powders, comparing (1) a blend of the elemental powders, (2)spray-dried agglomerate, and (3) plasma-densified spray-driedagglomerate (i.e., densified composite particles). In this exemplaryembodiment, the alloy contains 90% W, 5% Ni, 3% Fe, and 2% Cu (byweight) and has a theoretical bulk density of about 17.0 g/cc. Invarious embodiments, the plasma power of the plasma jet is reduced inorder to avoid volatilization of one or more constituents of the alloyagglomerate particles, e.g., those elements having the lowest meltingpoints. For example, these exemplary tungsten heavy metal alloycomposite particles were plasma densified with a plasma power of about28 kW, compared to a plasma power of 36 kW that is typically utilized toplasma densify molybdenum powder.

As shown, the blend of the elemental powders is not flowable and thusnot suitable for additive manufacturing in accordance with embodimentsof the present invention. As also shown, plasma densification results incomposite particles having a much higher bulk density than the densityof the powder after blending or after spray drying. As utilized herein,“bulk density” of a powder corresponds to the collective weight of theparticles of the powder divided by the volume occupied by suchparticles. In various embodiments, plasma-densified composite particleshave bulk densities that are within approximately 35% of, withinapproximately 40% of, within approximately 45% of, within approximately50% of, within approximately 55% of, within approximately 60% of, oreven within approximately 65% of that of the desired alloy. In variousembodiments, the bulk density of plasma-densified composite particles isless than approximately 75% of, less than approximately 70% of, or lessthan approximately 65% of that of the desired alloy.

Plasma densification may also reduce the particle-size distribution ofthe composite particles, even in the absence of separate particlefiltering or classification, as shown in the table above, where aparticle-size distribution dX of Y denotes that X % of particles have asize less than Y. In various embodiments, the densified compositeparticles may have a particle-size distribution d10 between 1 micronsand 10 microns, between 2 microns and 8 microns, between 4 microns and 8microns, or between 5 microns and 7 microns. In various embodiments, thedensified composite particles may have a particle-size distribution d50between 10 microns and 40 microns, between 10 microns and 25 microns,between 15 microns and 40 microns, or between 15 microns and 25 microns.In various embodiments, the densified composite particles may have aparticle-size distribution d90 between 40 microns and 80 microns,between 40 microns and 70 microns, between 50 microns and 70 microns, orbetween 60 microns and 70 microns.

FIG. 3A is a micrograph depicting the spray-dried agglomerate particlesdescribed above in relation to the above table, while FIG. 3B is amicrograph depicting the plasma-densified composited particles describedabove in relation to the above table. As shown, a significant percentageof the agglomerate particles in FIG. 3A are not substantially spherical.In contrast, the densified composite particles are substantiallyspherical. Furthermore, the densified composite particles appear to beless porous. In addition, as shown in FIG. 4, the densified compositeparticles 400 each include, consist essentially of, or consist of acollection of tungsten grains 410 that are surrounded by a matrix 420that includes, consists essentially of, or consists of a mixture oralloy of one or more of (or even all of) the other elements present inthe desired tungsten heavy metal alloy—Ni, Fe, and Cu in the depictedexample. Since the non-tungsten constituents of tungsten heavy alloystend to have melting points lower than that of tungsten, they are moresusceptible to melting during densification and therefore form thematrix surrounding the higher-melting-point tungsten grains in each ofthe densified composite particles 400.

In accordance with embodiments of the invention, the alloy compositeparticles (and/or the non-plasma-densified agglomerate particles) areutilized to form a 3D part by additive manufacturing. In an exemplaryembodiment, a printing apparatus 500 is utilized to fabricate a part inaccordance with embodiments of the invention, as shown in FIG. 5. Asshown, a printing head 505 is utilized to disperse a liquid binder oradhesive 510 (which may include, consist essentially of, or consist of,e.g., one or more polymeric materials) into a powder bed 515 that may becontained within a build container 520. The powder bed 515 contains theparticles of the desired metal alloy (e.g., densified compositeparticles and/or spray-dried agglomerate particles), and the printinghead 505 disperses the liquid binder layer by layer in approximately thedesired shape and size for the final desired part 525 (shown in FIG. 5as partially defined via dispersal of multiple layers of binder 510 intothe powder bed 515). After each layer of the binder 510 is dispersedover the powder bed 515, the layer of binder 510 is typically cured by,e.g., application of heat and/or light (e.g., infrared light orultraviolet light). For example, a curing head 530 may be moved over thepowder bed 515 while emitting heat and/or light onto the layer of binder510. As such, the curing head 530 may include, consist essentially of,or consist of one or more heaters (e.g., resistive heaters) and/or lightsources (e.g., lamps, light-emitting diodes, lasers, etc.). After eachlayer of the binder 510 is cured, another layer of the powder particlesis dispensed into the powder bed 515 over the partially completed partby a powder dispersal head 535, and the process is repeated. After eachlayer, the powder bed 515 may be translated vertically (i.e., in the “z”direction) relative to the printing head 505, the curing head 530, andthe powder dispersal head 535 (i.e., any of the tools and/or the powderbed may be translated) to accommodate the growing part.

The printing head 505, the curing head 530, and the powder dispersalhead 535 may move over the powder bed 515 in one, two, or threedimensions. As shown in FIG. 5, these tools may move via a gantry 540 orother framework suspended over the powder bed 515, for example, usingone or more motors, actuators, and/or stepper motors. Although FIG. 5depicts these tools as separate and independently movable, in variousembodiments the printing head 505, the curing head 530, and the powderdispersal head 535 are portions of a single unified fabrication headthat is movable over the powder bed and dispenses binder, cures thebinder, and dispenses powder from different portions thereof.Arrangements enabling the motion of the dispersal and curing toolsrelative to the powder bed are known in the printing, plotting, andscanning arts and may be provided by one of skill in the art withoutundue experimentation. In other embodiments, the build container 520itself may be moved relative to the tools in addition to or instead ofone or more of the tools moving. Such relative movement may becontrolled by a computer-based controller 545 based on electronicallystored representations of the part to be fabricated. For example, thetwo-dimensional layers traced out by the printing head 505 may beextracted from a stored three-dimensional representation of the finalpart stored in a memory 550.

The computer-based control system (or “controller”) 545 in accordancewith embodiments of the present invention may include or consistessentially of a general-purpose computing device in the form of acomputer including a processing unit (or “computer processor”) 555, thesystem memory 550, and a system bus 560 that couples various systemcomponents including the system memory 550 to the processing unit 555.Computers typically include a variety of computer-readable media thatcan form part of the system memory 550 and be read by the processingunit 555. By way of example, and not limitation, computer readable mediamay include computer storage media and/or communication media. Thesystem memory 550 may include computer storage media in the form ofvolatile and/or nonvolatile memory such as read only memory (ROM) andrandom access memory (RAM). A basic input/output system (BIOS),containing the basic routines that help to transfer information betweenelements, such as during start-up, is typically stored in ROM. RAMtypically contains data and/or program modules that are immediatelyaccessible to and/or presently being operated on by processing unit 555.The data or program modules may include an operating system, applicationprograms, other program modules, and program data. The operating systemmay be or include a variety of operating systems such as MicrosoftWINDOWS operating system, the Unix operating system, the Linux operatingsystem, the Xenix operating system, the IBM AIX operating system, theHewlett Packard UX operating system, the Novell NETWARE operatingsystem, the Sun Microsystems SOLARIS operating system, the OS/2operating system, the BeOS operating system, the MACINTOSH operatingsystem, the APACHE operating system, an OPENSTEP operating system oranother operating system of platform.

Any suitable programming language may be used to implement without undueexperimentation the functions described herein. Illustratively, theprogramming language used may include assembly language, Ada, APL,Basic, C, C++, C*, COBOL, dBase, Forth, PYTHON, FORTRAN, Java, Modula-2,Pascal, Prolog, Python, REXX, and/or JavaScript for example. Further, itis not necessary that a single type of instruction or programminglanguage be utilized in conjunction with the operation of systems andtechniques of the invention. Rather, any number of different programminglanguages may be utilized as is necessary or desirable.

The computing environment may also include other removable/nonremovable,volatile/nonvolatile computer storage media. For example, a hard diskdrive may read or write to nonremovable, nonvolatile magnetic media. Amagnetic disk drive may read from or writes to a removable, nonvolatilemagnetic disk, and an optical disk drive may read from or write to aremovable, nonvolatile optical disk such as a CD-ROM or other opticalmedia. Other removable/nonremovable, volatile/nonvolatile computerstorage media that can be used in the exemplary operating environmentinclude, but are not limited to, magnetic tape cassettes, flash memorycards, digital versatile disks, digital video tape, solid state RAM,solid state ROM, and the like. The storage media are typically connectedto the system bus through a removable or non-removable memory interface.

The processing unit 555 that executes commands and instructions may be ageneral-purpose computer processor, but may utilize any of a widevariety of other technologies including special-purpose hardware, amicrocomputer, mini-computer, mainframe computer, programmedmicro-processor, micro-controller, peripheral integrated circuitelement, a CSIC (Customer Specific Integrated Circuit), ASIC(Application Specific Integrated Circuit), a logic circuit, a digitalsignal processor, a programmable logic device such as an FPGA (FieldProgrammable Gate Array), PLD (Programmable Logic Device), PLA(Programmable Logic Array), RFID processor, smart chip, or any otherdevice or arrangement of devices that is capable of implementing thesteps of the processes of embodiments of the invention.

After the final layer of the binder 510 has been dispersed and cured andprinting is complete, the shaped, 3D part is composed of the particlesheld together by the cured binder material. The shaped part may then besintered to fuse the particles together and melt away some or all of thebinder material and possibly leave empty pores (if desired; such poresmay subsequently be infiltrated with another material via, e.g.,dispersing the material (e.g., one or more metals) in powder or liquidform on the part and sintering). For example, the part may be sinteredfor times up to approximately 1 hour, or even longer, and a temperaturesranging from approximately 1200° C. to approximately 1600° C. (e.g.,from approximately 1400° C. to approximately 1500° C.). The sinteringmay be performed at low pressure (e.g., at least partial vacuum) or in areducing (e.g., hydrogen or hydrogen-containing) atmosphere. Thesintering process may also result in densification of the particles andshrinkage of the part, particularly in embodiments in which theparticles are not further densified (e.g., by plasma densification)after spray drying. (As detailed herein, the use of densified compositeparticles for the additive manufacturing process advantageously resultsin, in various embodiments, reduced or minimal shrinkage of the finishedpart during any final sintering process.) The sintering may be performedbelow the melting point of a primary element within the particles andabove the melting points of one or more secondary elements within theparticles at lesser amounts. For example, in embodiments of theinvention in which the 3D part includes, consists essentially of, orconsists of a tungsten heavy alloy (or other refractory metal alloy),the sintering may be performed at a temperature approximately equal toor exceeding the melting point of one or more of the non-tungsten (ornon-refractory-metal) elements within the alloy and below the meltingpoint of tungsten (or the other refractory metal(s)). Pressure may alsobe applied to the part during sintering; thus, the part may be sinteredwithin a hot isostatic pressing process. Alternatively or in addition,the part may be cold isostatically pressed or hot isostatically pressedafter sintering.

FIG. 6 depicts a magnified view of part of an object produced byadditive manufacturing of spray-dried tungsten heavy alloy agglomerateparticles and sintered at approximately 1450° C. for 1 hour in ahydrogen atmosphere, in accordance with an embodiment of the invention.The table below compares various properties of the 3D-printed andsintered part with requirements of a relevant ASTM specification fortungsten heavy alloys (see ASTM B777-07(2013), Standard Specificationfor Tungsten Base, High-Density Metal, ASTM International, WestConshohocken, Pa., 2013, the entire disclosure of which is incorporatedby reference herein.) As shown, the density and hardness of the partmeet the ASTM specification, even though additional densification of theagglomerate particles was not performed before additive manufacturing.The composition of the agglomerate particles was 90% W, 5% Ni, 3% Fe,and 2% Cu.

The table below compares various properties of two other 3D-printed andsintered parts with the same ASTM standard, where these parts werefabricated using tungsten heavy alloy composite particles having twodifferent compositions, Composition A (91% W, 5% Ni, 2.5% Fe, and 1.5%Cu), and Composition B (91.5% W, 6.5% Ni, and 2% Fe), and that wereplasma densified prior to additive manufacturing.

0.2% Den- Hard- Yield Elon- Shrink- sity ness UTS Strength gation age(g/cc) (HRc) (MPa) (MPa) (%) (%) ASTM B777 16.85- 32 max 758 min 517 min5 min 18 Spec. - Class 17.25 1 Blended Material Composition A 17.24 27.3770 531 8.6 18-20 Composition B 17.22 24.1 878 564 25.1 18-20The shrinkage of these parts after printing and subsequent sintering wasonly 18%-20%, compared to the 30%-38% shrinkage resulting in partsprinted utilizing agglomerate particles that are spray dried but notplasma densified. As shown in the table, the characteristics of theseparts meets or exceeds the relevant ASTM standard. In addition, partsprinted utilizing plasma densified composite particles have been foundto have higher green strengths than those printed utilizing agglomerateparticles that are spray dried but not plasma densified. FIG. 7 showsthe microstructure of the printed and sintered part fabricated from thespray dried and plasma densified powder particles having Composition A.The microstructure in FIG. 7 is substantially identical to themicrostructure of a tungsten heavy alloy metal part fabricated utilizingconventional powder metallurgy techniques.

In additional embodiments of the invention, the spray-dried agglomerateparticles and/or the plasma-densified composite particles are fabricatedinto a wire that is subsequently utilized as feedstock for additivemanufacturing. For example, the agglomerate and/or composite particlesmay be placed within a tube or a cylindrical mold to form a wirepreform. In embodiments utilizing a tube, the tube may include, consistessentially of, or consist of at least one of the elements of thedesired alloy (and thus present within the particles), and the tube maybecome part of the final wire after drawing. In various embodiments,some or all of the particles of the preform are substantially sphericalplasma-densified particles as detailed herein. In various embodiments,such substantially spherical particles may be mixed with non-sphericalparticles to form the preform and, eventually, the wire, as detailed inU.S. patent application Ser. Nos. 15/416,253 and 15/416,254, filed onJan. 26, 2017, the entire disclosure of each of which is incorporatedherein by reference.

In various embodiments, the preform may be further densified beforefurther processing into wire. For example, the preform may be pressedby, e.g., hot isostatic pressing or cold isostatic pressing. Afterformation of the preform, the preform is processed into a wire. In anexemplary embodiment, the preform is formed into wire via drawingthrough one or more drawing dies until the diameter of the wire isreduced to the desired dimension. In various embodiments, the drawing issupplemented with or replaced by one or more other mechanicaldeformation processes that reduce the diameter (or other lateraldimension) of the preform, e.g., pilgering, rolling, swaging, extrusion,etc. The preform and/or wire may be annealed during and/or afterdiameter reduction (e.g., drawing).

In various embodiments, the preform is formed of the agglomerate and/orcomposite particles within a tube that includes, consists essentiallyof, or consists of one or more of the elements of the desired metalalloy (e.g., a refractory metal alloy such as a tungsten heavy alloy).The tube may itself be coaxially disposed within one or more other tubesthat include, consist essentially of, or consist of one or more otherelements of the alloy. When the preform containing the one or more tubesis drawn down into wire, the cross-section of the wire will thus includeall of the elemental constituents of the desired alloy. In variousembodiments, at least a portion of the powder particles may be furtherdensified before being placed into the tube(s). For example, the powderparticles may be pressed by, e.g., hot isostatic pressing or coldisostatic pressing.

In various embodiments, the one or more tubes may include, consistessentially of, or consist of one or more elements that are more ductilethan one or more of the elements present in powder form. For example,the one or more tubes may include, consist essentially of, or consist ofNb, Ta, Ti, and/or Zr. In other embodiments, the one or more tubes mayinclude, consist essentially of, or consist of one or more of thenon-refractory-metal elements in the desired alloy (e.g., one or moretransition metals). In various embodiments, the one or more tubes have asufficiently small diameter that the preform itself may be utilized asthe final wire without further processing or diameter reduction such aswire drawing. In various embodiments, the one or more tubes, with theparticles therewithin, may be annealed and/or subjected to pressure(e.g., hot-isostatically pressed) before (or between multiple steps of)the process of diameter reduction. Such treatment may advantageouslyreduce void space within and increase the density of the final wire.

In other embodiments, the preform may feature a sacrificial tube inwhich the particles are disposed. After processing of the preform intowire, the sacrificial tube may be etched or melted away, and the finalwire includes, consists essentially of, or consists of the elements ofthe desired alloy arising solely from the original particles. In variousembodiments, one or more tubes to be processed as part of the wire maybe disposed within the sacrificial tube; at least portions of such tubeswill typically remain as portions of the wire after removal of thesacrificial tube. The sacrificial tube may include, consist essentiallyof, or consist of, for example, plastic, rubber, one or more polymericmaterials, a metallic material having a melting point lower than one ormore (or even all) of the metallic elements within the particles, ametallic material selectively etchable (i.e., over the metallic elementswithin the particles and other tubes), etc.

Once the wire including, consisting essentially of, or consisting of thedesired alloy (e.g., a tungsten heavy alloy or other refractory-metalalloy) is fabricated in accordance with embodiments of the invention,the wire may be utilized to fabricate a three-dimensional part with anadditive manufacturing assembly 800. For example, as shown in FIG. 8,the wire may be incrementally fed, using a wire feeder 810, into thepath of a high-energy source 820 (e.g., an electron beam or a laser beamemitted by a laser or electron-beam source 830), which melts the tip ofthe wire to form a small molten pool (or “bead” or “puddle”) 840. Theentire assembly 800 may be disposed within a vacuum chamber to preventor substantially reduce contamination from the ambient environment.

Relative movement between a substrate 850 (which may be, as shown,disposed on a platform 860) supporting the deposit and the wire/gunassembly results in the part being fabricated in a layer-by-layerfashion. Such relative motion results in the continuous formation of alayer 870 of the three-dimensional object from continuous formation ofmolten pool 840 at the tip of the wire. As shown in FIG. 8, all or aportion of layer 870 may be formed over one or more previously formedlayers 880. The relative movement (i.e., movement of the platform 860,the wire/gun assembly, or both) may be controlled by computer-basedcontroller 545 based on electronically stored representations of thepart to be fabricated. For example, the two-dimensional layers tracedout by the melting wire may be extracted from a stored three-dimensionalrepresentation of the final part stored in memory 550. After theadditive manufacturing process is complete, the part may be removed fromthe platform and subjected to final machining and/or polishing.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

What is claimed is:
 1. A powder for additive manufacturing of a partcomprising a tungsten heavy alloy, wherein the tungsten heavy alloy (i)comprises 90% or more tungsten and 10% or less of one or more additionalelements selected from the group consisting of nickel, iron, copper,cobalt, and manganese, and (ii) has a theoretical density correspondingto a weighted average of the densities of tungsten and the one or moreadditional elements, the powder comprising a plurality of substantiallyspherical composite particles, wherein: each composite particlecomprises a plurality of tungsten grains surrounded by a matrixcomprising the one or more additional elements, a bulk density of thepowder is approximately 35% or more of the theoretical density, and atap density of the powder ranges from approximately 40% to approximately75% of the theoretical density.
 2. The powder of claim 1, wherein a Hallflow rate of the powder ranges from approximately 1 s/50 g toapproximately 15 s/50 g.
 3. The powder of claim 1, wherein the powderhas a particle-size distribution d10 between 2 microns and 8 microns,d50 between 15 microns and 25 microns, and d90 between 50 microns and 70microns, wherein a particle-size distribution dX of Y denotes that X %of particles have a size less than Y.
 4. The powder of claim 1, whereinthe bulk density of the powder is approximately 45% or more of thetheoretical density.
 5. The powder of claim 1, wherein the bulk densityof the powder is approximately 50% or more of the theoretical density.6. The powder of claim 1, wherein the bulk density of the powder isapproximately 65% or less of the theoretical density.
 7. A method offorming a powder comprising a tungsten heavy alloy, wherein the tungstenheavy alloy (i) comprises 90% or more tungsten and 10% or less of one ormore additional elements selected from the group consisting of nickel,iron, copper, cobalt, and manganese, and (ii) has a theoretical densitycorresponding to a weighted average of the densities of tungsten and theone or more additional elements, the method comprising: forming a powderblend by blending together powders of tungsten and the one or moreadditional elements; forming a slurry by mixing the powder blend with aliquid, the liquid comprising water and/or one or more organic binders;spraying the slurry and a heated gas into a drying chamber to form aplurality of agglomerate particles each comprising a mixture of tungstenand the one or more additional elements; and densifying at least aportion of the plurality of agglomerate particles to form the powder,the densifying comprising heating to a temperature greater than amelting point of at least one of the additional elements and less than amelting point of tungsten, wherein the powder comprises a plurality ofsubstantially spherical composite particles, each composite particlecomprising a plurality of tungsten grains surrounded by a matrixcomprising the one or more additional elements.
 8. The method of claim7, wherein densifying comprises feeding the at least a portion of theplurality of agglomerate particles through a plasma.
 9. The method ofclaim 7, wherein a Hall flow rate of the powder ranges fromapproximately 1 s/50 g to approximately 15 s/50 g.
 10. The method ofclaim 7, wherein the powder has a particle-size distribution d10 between2 microns and 8 microns, d50 between 15 microns and 25 microns, and d90between 50 microns and 70 microns, wherein a particle-size distributiondX of Y denotes that X % of particles have a size less than Y.
 11. Themethod of claim 7, wherein the bulk density of the powder isapproximately 45% or more of the theoretical density.
 12. The method ofclaim 7, wherein the bulk density of the powder is approximately 50% ormore of the theoretical density.
 13. The method of claim 7, wherein thebulk density of the powder is approximately 65% or less of thetheoretical density.